This invention relates to a television signal synchronizing apparatus.
A television signal synchronizing apparatus synchronizes an input television signal with a read-out reference signal. The input television signal is converted into a digital video signal, and the digital video signal is written into a memory, having a write-in port and a readout port, at address locations produced by a write-in address generator, in synchronism with the input television signal. At the read-out port, the digital video signal is read out in accordance with read-out addresses generated in synchronism with the read-out reference signal. Thereafter, the read out digital video signal is converted back into an analog television signal. Such television signal synchronizing apparatus is disclosed in U.S. Pat. No. 4,007,486, for example.
On the other hand, the following relationships are defined between the color subcarrier frequency F.sub.sc, the horizontal sync frequency F.sub.H, the vertical sync frequency F.sub.V, and the frame frequency F.sub.F in the NTSC system: EQU F.sub.sc =(455/2).multidot.F.sub.H EQU F.sub.H =(525/2).multidot.F.sub.V EQU F.sub.F =1/2F.sub.V
The above equations imply that the phase of the color subcarrier is inverted 180 degrees between a given frame and the next frame. These consecutive two frames are defined as color frames I and II, respectively, hereinafter. In other words, the phase relationship between the color subcarrier and the horizontal sync signal completes one cycle within four fields. These consecutive fields are also defined as color fields I, II, III and IV, respectively, hereinafter.
In a television signal synchronizing apparatus having one frame memory capacity, i.e., a frame synchronizer, the relationship between the phase of the video signal and the phase of the horizontal sync signal in an input television signal is maintained in a read-out television signal (zero offset) when the color frame of the input television signal to be written into the frame memory agrees with that of the read-out reference signal. But when the relationships between the phase of the video signal and the phase of the horizontal sync signal in the input and read-out video signals are different, an undesirable phase shift (offset) of .+-.1/2 the color subcarrier period (.+-.140 ns) occurs between the phase of the video signal and the phase of the horizontal sync signal in the input and read out video signals. This happens when, for example, the television signal of the color frame I (color fields I and II) is read out under the control of color frame II (color fields III and IV) or vice versa. Therefore, the phase offset reaches a maximum of one subcarrier period (280 nsp-p) and causes horizontal picture shift on the television screen. In practice, of course, a composite sync signal in accordance with the read-out reference signal is added to the read-out video signal and, thus, a television signal regulated under the standard is delivered. But the offset between the position (phase) of the video signal and the horizontal sync position (phase) is still present.